1. Field of the Invention
The present invention generally relates to the field of semiconductor transistor devices, and more particularly to a method of manufacturing silicon nitride spacer-less semiconductor NMOS and PMOS transistor devices having improved saturation current (Idsat).
2. Description of the Prior Art
High-speed metal-oxide-semiconductor (MOS) transistor devices have been proposed in which a strained silicon (Si) layer, which has been grown epitaxially on a Si wafer with a silicon germanium (SiGe) layer disposed therebetween, is used for the channel area. In this type of strained Si-FET, a biaxial tensile strain occurs in the silicon layer due to the SiGe which has a larger lattice constant than Si, and as a result, the Si band structure alters, the degeneracy is lifted, and the carrier mobility increases. Consequently, using this strained Si layer for a channel area typically enables a 1.5 to 8-fold speed increase.
FIGS. 1-3 are schematic cross-sectional diagrams illustrating a prior art method of fabricating a semiconductor NMOS transistor device 10. As shown in FIG. 1, the conventional NMOS transistor device 10 generally includes a semiconductor substrate generally comprising a silicon layer 16 having a source 18 and a drain 20 separated by a channel region 22. The silicon layer 16 is typically a strained silicon layer formed by epitaxially growing a silicon layer on a SiGe layer (not shown). Ordinarily, the source 18 and drain 20 further border a shallow-junction source extension 17 and a shallow-junction drain extension 19, respectively. A thin oxide layer 14 separates a gate 12, generally comprising polysilicon, from the channel region 22.
In the device 10 illustrated in FIG. 1, the source 18 and drain 20 are N+ regions having been doped by arsenic, antimony or phosphorous. The channel region 22 is generally boron doped. A silicon nitride spacer 32 is formed on sidewalls of the gate 12. A liner 30, generally comprising silicon dioxide, is interposed between the gate 12 and the silicon nitride spacer 32. A salicide layer 42 is selectively formed on the exposed silicon surface of the device 10. Fabrication of an NMOS transistor such as the device 10 illustrated in FIG. 1 is well known in the art and will not be discussed in detail herein.
Referring to FIG. 2, after forming the NMOS transistor device 10, a silicon nitride cap layer 46 is typically deposited thereon. As shown in FIG. 2, the silicon nitride cap layer 46 covers the salicide layer 42 and the silicon nitride spacer 32. The thickness of the silicon nitride cap layer 46 is typically in the range of between 200 angstroms and 400 angstroms for subsequent etching stop purposes. A dielectric layer 48 such as silicon oxide or the like is deposited over the silicon nitride cap layer 46. The dielectric layer 48 is typically much thicker than the silicon nitride cap layer 46.
Referring to FIG. 3, subsequently, conventional lithographic and etching processes are carried out to form a contact hole 52 in the dielectric layer 48 and in the silicon nitride cap layer 46. As aforementioned, the silicon nitride cap layer 46 acts as an etching stop layer during the dry etching process to alleviate source/drain damages caused by the etchant substances.
However, prior art techniques involving the deposition of a graded SiGe layer underneath the silicon channel have several drawbacks. The SiGe layer tends to introduce defects, sometimes called threading dislocations, in the silicon, which can impact yields significantly. Also, the graded SiGe layer is deposited across the wafer, making it harder to optimize the NMOS and PMOS transistors separately. And the silicon germanium layer has poor thermal conductivity. Another concern with the conventional approach is that some dopants diffuse more rapidly through the SiGe layer, resulting in a non-optimium diffusion profile in the source/drain regions.
Thus, a need exists in this industry to provide an inexpensive method for making a MOS transistor device having improved functionality and performance.